1. Background Art
Electromagnetic interference has increased in recent years due to advances in the performance of electrical and electronic equipment and the growing use of the same. Even such displays as CRTs and PDPs emit electromagnetic waves. A PDP is an assembly composed of a glass substrate having a data electrode and a fluorescent layer, and a glass substrate having a transparent electrode. When operated, such a display not only emits visible light that produces an image, but also generates electromagnetic waves, near infrared rays, and heat in large amounts. In general, a front panel containing an electromagnetic wave shielding sheet is mounted on the front of a PDP in order to shield electromagnetic waves. The required efficiency in shielding electromagnetic waves with frequencies of 30 MHz to 1 GHz, emitted from the front of the display, is 30 dB or more.
Further, to make an image displayed on the display highly visible, the electromagnetic wave shielding material is required to be not highly visible (or highly invisible), and the front panel is required to have moderate transparency (visible light transmission), as a whole.
Furthermore, PDPs are characterized by having large-sized screens, and electromagnetic wave shielding sheets for such PDPs are large in size (external dimension); their sizes are as large as 621×831 mm for 37-inch displays and 983×583 mm for 42-inch displays, for example, and still larger sizes exist. This fact requires a production process that is convenient to handle large-sized materials. There is, therefore, a demand for a process for producing an electromagnetic wave shielding sheet by which an electromagnetic wave shielding sheet can be obtained in a small number of steps with high productivity.
2. Prior Art
Electromagnetic wave shielding sheets containing metal mesh layers have so far been known as electromagnetic wave shielding sheets having visible light transmission and electromagnetic wave shielding ability that are compatible with each other. Usually used to produce such electromagnetic wave shielding sheets are the following two processes.
A known process for producing an electromagnetic wave shielding sheet of the above-described type is that electrically conductive ink or a photosensitive coating liquid containing a catalyst for chemical plating is applied to the entire surface of a transparent substrate, and the coating is photolithographically made into a mesh, which is then plated with a metal (see Patent Documents 1 and 2, for example). However, such a metal mesh layer is disadvantageous in that, since it is highly lustrous and reflects extraneous light such as sunlight, the mesh is noticeable, and, moreover, a screen appears white due to the metal mesh layer to decrease image contrast. In order to solve the above problem, a blackening layer is formed on the observer-side surface of the metal mesh layer. However, so far as this means is employed, it is impossible to blacken the transparent-substrate-side surface of the metal layer. Therefore, this means cannot cope with such a design that an electromagnetic wave shielding sheet is mounted on a display with the transparent substrate side of the sheet facing to the observer side. In addition, it takes a long time for plating if electrically conductive ink is used because such ink has high electrical resistance.
Another known process for producing an electromagnetic wave shielding sheet of the above-described type is as follows: a PET film (transparent substrate) and a copper mesh layer with line parts that define multiple openings are laminated with an adhesive layer; and all of the surfaces, back surfaces, and side faces of the line parts of the copper mesh layer are subjected to blackening treatment (see Patent Document 3, for example). An electromagnetic wave shielding sheet obtained by this process absorbs all extraneous light incident on the surfaces, back surfaces, and side faces of the line parts of the mesh, and all light entering the back surfaces and side faces of the line parts from a display, so that it is possible to prevent glistening and whitening of the line parts of the mesh that occurs when the line parts reflect the light. It is, therefore, possible to prevent, even in the light, the mesh from glistening and whitening to become noticeable and also a screen from appearing white to make image contrast lower. In this production process, however, it is necessary to form, on the entire metal mesh surface, a layer of another material such as copper oxide by such a method as plating, so that the blackening layer thus formed has been disadvantageous in that it easily falls off the metal. Further, since it is necessary to prepare a material for forming the blackening layer, material cost increases, and, moreover, only limited materials can be used for this purpose when adhesion to the metal, etc. are taken into consideration. Furthermore, it is necessary to add, to the production process, the step of conducting anticorrosion treatment for imparting corrosion resistance to the metal layer, and the addition of this step reduces productivity.
Patent Document 1: Japanese Laid-Open Patent Publication No. 13088/2000,
Patent Document 2: Japanese Laid-Open Patent Publication No. 59079/2000, and
Patent Document 3: Japanese Laid-Open Patent Publication No. 9484/2002.